1. Field of the Invention
The invention relates to methods and tools used in knee arthroplasty. More particularly, the invention relates to methods and tools used in revision surgery where an artificial femoral component is removed and replaced.
2. Brief Description of the Prior Art
Total knee arthroplasty involves the replacement of portions of the patellar, femur and tibia with artificial components. In particular, a proximal portion of the tibia and a distal portion of the femur are cut away (resected) and replaced with artificial components. As used herein, when referring to bones or other body parts, the term "proximal" means closest to the heart and the term "distal" means more distant from the heart. When referring to tools and instruments, the term "proximal" means closest to the practitioner and the term "distal" means distant from the practitioner.
There are several types of knee prostheses known in the art. One type is sometimes referred to as a "resurfacing type". In these prostheses, the articular surface of the distal femur and proximal tibia are "resurfaced" with respective metal and plastic condylar-type articular bearing components. These knee prostheses provide adequate rotational and translational freedom and require minimal bone resection to accommodate the components within the boundaries of the available joint space.
The femoral component is a metallic alloy construction (cobalt-chrome alloy or 6A14V titanium alloy) and provides medial and lateral condylar bearing surfaces of multi-radius design of similar shape and geometry as the natural distal femur or femoral-side of the knee joint.
The tibial component usually includes a distal metal base component and a proximal interlocking plastic, e.g. UHMWPE (ultra high molecular weight polyethylene), component or insert. The plastic tibial plateau bearing surfaces are of concave multi-radius geometry to more or less match the articular geometry of the mating femoral condyles. Both the femoral and tibial components are usually provided with intermedullary (IM) stem options.
After preparing the distal surface of the femur and the proximal surface of the tibia, an opening is made into the medullary canal of the femur, and an opening is made into the medullary canal of tibia. The interior surface and the IM stem of the femoral component are usually covered with a polymeric cement and the IM stem is inserted into the medullary canal of the femur until the interior surface of the femoral component meets the distal surface of the femur. The tibial component is similarly usually cemented to the proximal surface and medullary canal of the tibia.
Occasionally, the components are press fit without the use of cement. The use of cement has advantages and disadvantages. Press fit components rely on bone quality to obtain good fixation. Sometimes it is impossible to obtain good fixation with a press fit component and sometimes a press fit component will fail early because of failure of successful biological ingrowth. Cement assures good fixation but puts strain along the component stem. In addition, as described below, cement complicates the removal of a failed component.
Often, due to normal wear over time, the prosthetic knee must be replaced via a procedure known as revision surgery. When the primary cemented prosthetic is removed, the proximal surface of the tibia and the distal surface of the femur typically exhibit cavernous defects. Absent the use of bone graft, the proximal surface of the tibia and the distal surface of the femur must be carefully resected to remove cavernous defects before a replacement knee can be installed.
In addition, the intramedullary (IM) canals must be broached or reamed to remove any remaining cement or cavernous defects existing in the canals before a replacement knee can be installed. In many instances, the replacement femoral component will be provided with a posterior stabilizer and a posterior distal portion of the femur will need to be removed in order to accommodate the posterior stabilizer.
The absence of bony landmarks (removed during primary surgery) and the presence cavernous defects make extramedullary (EM) alignment of cutting jigs difficult even in cases where the primary prosthetic did not use cement.
The state of the art method for accomplishing revision arthroplasty involves the use of several cutting blocks which must be aligned with reference to the IM canal.
After the primary prosthetic is removed, the distal femur is resected with a lateral template. The medullary canal is reamed and the reamer is tapped in place with a mallet. A distal resection guide is attached to the reamer and distal resection is completed via slots in the guide. The distal resection guide is removed from the reamer and another cutting block is attached to the reamer for A/P and chamfer resections.
The rotational alignment of the femoral component is critical to ensure correct patellar tracking. Since the posterior condyles are no longer present, this cutting block must be carefully aligned relative to the femoral epicondyles where the collateral ligaments are attached.
After anterior/posterior and chamfer resections are completed, the cutting block is removed and fourth cutting block is attached to the reamer in order to accomplish intercondylar box resection. It will be appreciated that the installation and removal of the several cutting blocks makes alignment of the cutting blocks more difficult.
Following preparation of the femur, similar procedures are performed on the proximal tibia. In particular, a reamer is installed with a mallet. An anterior resection block is pinned to the tibia and a proximal portion of the tibia is resected.
The defect in the tibia is measured and the cutting guide is moved down 6 to 10 mm. A flat cut from anterior to posterior is made. A tibial template is attached to the reamer and reference marks are made with a blue pen. A flat cut and sagittal cut are made relative to the reference marks. Another template is attached to the reamer and anterior and posterior holes are drilled for securing a wedge resection guide. A wedge cut is then made. The template is replaced and aligned with the marks. A revision mask punch guide is attached to the template and a revision box chisel is used to prepare for a stem. The femur and tibia are now in condition for trialing.
Trialing is accomplished by attaching a femoral trial augmentation and stem extension, tightening the stem extension into a stem boss, positioning a trial augmentation block on the underside of a trial plate, inserting bolts through the top of the plate and tightening the bolts, inserting a constrained modular post into the bearing trial, placing the constrained femoral trial, and stemmed tibial trial into the joint space. After successful trialing, the femoral and tibial components are installed.
Those skilled in the art will appreciate that revision surgery is difficult because (1) the type and location of cavernous defects make it difficult to match the exterior surfaces of the tibia and femur to the interior surfaces of the prosthetic, (2) the femur and tibia must be resected with reference to the IM canal, and (3) the use of multiple templates and guides during the course of the procedure makes it very difficult to keep all the cuts in proper alignment relative to the IM canal.